This application requests funds to purchase a cesium 137 irradiation source to operate within the barrier Animal Care Facility (ACF) at the Parnassus Heights campus of the University of California, San Francisco (UCSF). Parnassus Heights is the major site of teaching, research, and patient care at UCSF. Locating this instrument within the ACF will allow 11 co-investigators working in the areas of immunology, infectious disease, hematopoiesis, and cancer biology whose laboratories are located on the Parnassus campus to perform experiments which are integral to NIH-funded research projects and which involve transferring hematopoietic cells into irradiated mice. Acquisition of a cesium irradiator for the ACF will ensure that these studies are carried out efficiently, safely, and in a manner that follows well-established procedures for conducting working within the controlled environment of a barrier animal care facility. The applicant group has a excellent record of research productivity and a strong history of collaborating with each other. At present, there is no radiation source available for irradiating small animals at the Parnassus campus. Existing procedures are expensive, cumbersome, and inefficient as they involve housing mice at a different site from the investigator s laboratory or transferring animals to another site within San Francisco for radiation, and then transporting them back to the Parnassus campus. The instrument will be housed within the ACF, will add an important new component to the infrastructure of that facility, and will greatly enhance research efforts of UCSF investigators in addition to the applicant group. The application documents strong institutional support for the proposal.